The subject matter of the present invention relates to an improved core marking system for a borehole sidewall coring tool adapted for use in a wellbore.
Sidewall coring tools are used for the purpose of obtaining a sample of a formation traversed by a wellbore. In the following discussion, the terms "sample", "sidewall core", "core sample" and "core" are used interchangeably. In each sidewall coring tool, a marker system is used to mark each sample of the formation in order to obtain an indication of the depth of the sample in the wellbore. For example, U.S. Pat. No. 4,714,119 to Hebert et al, which issued Dec. 22, 1987 (hereinafter referred to as "the Hebert patent") is directed to a sidewall coring tool that is adapted for cutting and obtaining sidewall cores of a formation traversed by the borehole, the direction of the cut being perpendicular to an axis of the borehole. The disclosure of the Hebert patent is incorporated by reference into this specification. However, although the marker system used in connection with the Hebert patent has performed adequately, a need has arisen for an improved, more reliable marker system for use in connection with borehole sidewall coring tools.
The proper operation of a marker or indexing system is important because it is the principal method by which the retrieved sidewall samples are identified and correlated to the depths at which they were taken. Failure to properly identify the cores leads to the loss of all retrieved samples, since the interpretation, analysis and information concerning the retrieved samples is of value only when the correct depth of origin is known. If the depth of origin of one sample is unknown, the origins of all of the samples become subject to question.
Therefore, the potential for failure of the entire operation exists when the marking system malfunctions. In a horizontal wellbore, such a service might not even be attempted if it was believed that no indexing system would be present or available. Typical operational problems, encountered by the operators of a sidewall coring tool, are high borehole fluid density and high borehole fluid viscosity. In gravity feed marking systems, such as those described in U.S. Pat. Nos. 4,449,593 and 4,714,119, proper functioning of the tool relies on having the marker fall into a core storage tube or vessel driven exclusively by the force of gravity. However, fluid densities can be high (sometimes in excess of twice the density of water). As the difference in the fluid density and the marker's density decreases, the buoyancy of the marker increases, and the tendency of the marker to fall decreases. High fluid viscosity is a more significant problem when the viscosity is high. The fluid is essentially a thick gel, and the markers as described in U.S. Pat. Nos. 4,449,593 and 4,714,119 are being held in suspension by the high viscosity fluid. This leads to erroneous placement or lack of placement of markers and subsequent improper indexing of core samples. This combination of high fluid density and high viscosity, which is commonly encountered, can prevent the marker from dropping at all. In high viscosity conditions, the markers tend to stick to the marker kicker and may be retracted when the marker kicker retracts. Examples of marker kicker devices are shown in U.S. Pat. No. 4,714,119 (element 65, "kicker foot") and U.S. Pat. No. 4,449,593 (element 72, "wafer ejector"). The problems presented by borehole fluid conditions exist in both horizontal and vertical tool positions. All of the above problems have been routinely cited by operating field locations as problems which they encounter during field operations.
Another problem involves the debris which exists in and around the core storage area. Debris in the well bore can be present in the form of rock cuttings from the borehole drilling process left in suspension in the borehole fluid or rock fragments knocked loose from the borehole wall by the motion of the entire apparatus. In addition, the drilling of the sidewall sample itself produces debris. Debris obstructions in the area leading to the core storage area can prevent recovery of the sidewall sample. Further, debris can also impede the delivery of the marker to the core storage area if the debris accumulates in front of the marker itself. This prevents the marker from being moved to the proper position. In addition, debris inside the core storage tube occupies space which is designated for core storage, reducing the maximum number of samples which can be recovered.
It has been found that the recovery from an oil well can be substantially increased in some cases by making the wellbore horizontal in the section of the well which will produce the petroleum. Recent improvements in the methods and practices for drilling of wells with horizontal boreholes have allowed horizontal drilling to become much more common place than was previously the case. It is common practice to refer to the well bore deviation with reference to the surface of the earth, so that well bores perpendicular to the surface of the earth are said to be vertical. In the course of evaluation of these wells, it is expected that most wireline formation evaluation tools must be able to operate in a horizontal position. Positioning the tool horizontally presents a new set of problems in addition to those posed by borehole fluid conditions. The system, used by the devices described in U.S. Pat. Nos. 4,449,593 and 4,714,119, has two problems: first, horizontal positioning removes the gravity force required to move the marker into the core storage tube; in these systems, the marker can either fall sideways away from the funnel as it is moved by the marker kicking device or it could fall into the funnel, and, depending on the angular orientation of the tool, fall out of the funnel into the borehole; and second, with the tool mechanism in the horizontal position, pieces of segmented, broken or fragmented cores are lost as the core is being directed to the core tube by the core pusher assembly. For the purposes of evaluation and analysis of the core, it is desirable to have as much of the core sample as possible. In addition, pieces of the core which fall out could jam the mechanism and prevent core removal. Segmented, broken and fragmented cores are observed reasonably frequently during sidewall coring operations. The condition of the core cannot be predicted, nor can it be assumed that recovered cores will be in one piece since the reasons for broken cores are also varying and unpredictable.
Thus a properly functioning marking system is critical for wellsite operations in order to ensure that the sidewall coring tool can be considered for use in the maximum number of potential applications and in different situations. In addition, it is important in all situations that as much of the core be recovered as possible to allow for the optimal analysis of recovered samples.
As a result, the need exists for an improved core marking system for use with a sidewall coring tool, which core marking system will reliably mark, index, and separate both whole and fragmented sidewall core samples regardless of the deviation of the wellbore in which the sidewall coring tool is disposed.